(AP Photo/Evan Vucci, File). FILE - In this March 15, 2018, file photo, President Donald Trump talks with reporters during a meeting in the Oval Office of the White House in Washington. Trump is questioning the impartiality of Robert Mueller's investig...

The president has long been frustrated by the lengthy and intensifying probe and insists his 2016 presidential campaign did not collude with Russia to influence the election in his favor.